Lagarfljót also called Fljótið is a lake situated in the east of Iceland near Egilsstaðir. Its surface measures 53 square kilometers and it is 25 kilometers long; its greatest width is 2.5 kilometers and its greatest depth 112 meters. The River Lagarfljót flows through this lake. The 27 MW Lagarfossvirkjun hydropower station is located at its lower end.

The biggest forest in Iceland, Hallormsstaðaskógur is found near the lake as well as a waterfall, Hengifoss. Hengifoss, at 128 meters, is one of the tallest waterfalls in the country. Below it is another waterfall called Litlanesfoss.

As with the Scottish lake Loch Ness, a cryptid serpent, called Lagarfljótsormurinn by locals, is believed by some to live in the depths of Lagarfljót.